The present invention relates generally to fault current indicators for alternating current electrical distribution systems, and more particularly to fault indicators having circuitry for disabling the indicator for a predetermined period of time following initial energization of a monitored circuit.
Fault indicators of various types have been constructed for detecting faults in electrical power distribution systems. Such indicators include clamp-on type fault indicators, which clamp directly over cables in the system, and test point-type indicators, which are mounted over test points in the cables or associated connectors. Fault indicators of both types may be either of the manually reset type, wherein it is necessary that the indicator be physically reset following each fault, or of the automatically reset type, wherein a fault indication is reset upon restoration of line current. Examples of such fault indicators are found in products manufactured by E. O. Schweitzer Manufacturing Company of Mundelein, Ill., and in U.S. Pat. Nos. 4,063,171, 4,251,770 and 4,263,550 of the present inventor.
Since fault conditions in a distribution system may be transitory in nature, as, for example, when a tree branch momentarily contacts a high voltage conductor, distribution circuits are frequently provided with automatic reclosers. In such circuits, the occurrence of a transitory fault will result in the opening of the circuit breakers along the affected portion of the circuit. Any fault indicators installed along the faulted conductor will indicate a fault condition. After a brief interval, the circuit interrupter is automatically reclosed by the recloser. If following reclosure the current is within normal limits, as would be the case if the fault condition was transitory and thus no longer present, the circuit interrupter remains closed and power is distributed through the associated portion of the system. If the fault condition is not transitory, and hence is still present at the time of the automatic recloser cycling, the circuit breaker again trips and power is not supplied to the affected portion of the distribution system. Following a predetermined number of recloser cyclings, the circuit breaker typically will no longer automatically reclose, and power will be removed from the affected section of the distribution system until such time as the fault condition is corrected, and the circuit breaker is manually reset.
Upon each cycling of the automatic recloser, the momentary inrush or initial transient current in the system may exceed normal current limits in the circuit for a short period of time immediately following the recloser cycling. In fact, the inrush current may reach 10 to 20 times the normal steady state current of the conductor and may last for one-half cycle. Frequently, this results in fault indicators installed in circuit branches switched by the circuit interrupter but not affected by the original fault condition being tripped. Consequently, a series of false fault indications may be made in correctly functioning portions of the distribution system.
The present invention is directed toward means for preventing the false tripping of fault indicators by the momentary inrush current immediately following restoration of power, as by the operation of the automatic circuit recloser. To prevent such false tripping, a fault indicator is provided wherein the fault indicator is disabled for a period immediately following re-energization of the circuit monitored by the fault indicator.
Accordingly, it is a general object of the present invention to provide a new and improved fault indicator.
It is a more specific object of the present invention to provide a new and improved fault indicator which remains insensitive to momentary or transient inrush current following restoration of current in a monitored circuit.